An Accurate Methodology to Identify the Level of Aggregation in Solution by PGSE NMR Measurements:  The Case of Half-Sandwich Diamino Ruthenium(II) Salts

The utility of PGSE NMR measurements in determining hydrodynamic radii (r H) and volumes (V H) of small- and medium-size molecules (3 Å < r H < 6 Å) was evaluated by performing measurements for a variety of pure deuterated solvents and their solutions containing the internal standard TMSS [tetrakis(trimethylsilyl)silane] also in the presence of a variable concentration of 3BPh4. It was found that accurate r H and V H values can be obtained by introducing in the Stokes−Einstein equation (D t = kT/cπηr H) not only the correct values for temperature (T) and viscosity (η) but, particularly, that for the c factor. PGSE NMR measurements were then applied to an investigation of the aggregation tendency of complexes [Ru(η6-cymene)(R1R2NCH2CH2NR1R2)Cl]X (R1 = R2 = H, 1; R1 = H, R2 = H, 2; R1 = R2 = Me, 3; X- = PF6 - or BPh4 -) in both protic and aprotic solvents with a relative permittivity (εr) ranging from 4.81 (chloroform-d) to 32.66 (methanol-d 4). Compounds 1 and 2 exhibited a remarkable tendency to aggregate through intercationic N−H···Cl and cation/anion N−H···FPF5 - hydrogen bonds. In addition to ion pairs, ion triples and ion quadruples were also observed in solution. Compound 3, having no N-H moiety, showed less tendency to aggregate than 1 and 2, even though it also afforded ion quadruples in apolar and aprotic solvents. Relative anion−cation orientations and arene conformations were investigated by means of 1H−NOESY and 19F,1H-HOESY NMR spectroscopy. The relative anion−cation position was well-defined, especially for compounds bearing the PF6 - counterion, and was modulated by the nature of the N,N ligand. A progressive slackening of the contact aggregates was observed in the series 1−3 that led to a higher mobility of the anion, as indicated by the observation of less specific interionic NOEs.